This invention relates to a relaxation oscillator comprising a capacitor, a charging circuit, a discharging circuit, switching means having a first position for charging the capacitor by means of the charging circuit and having a second position for discharging the capacitor by means of the discharging circuit, and a feedback circuit which supplies a first feedback signal for setting the switching means from the first position to the second position after the capacitor voltage has reached a first value and which supplies a second feedback signal for setting the switching means from the second position to the first position after the capacitor voltage has reached a second value.
A relaxation oscillator of this kind is known from the IEEE Journal of Solid State Circuits, Vol. SC-18, No. 6, Dec. 1983, pages 794-801. The feedback circuit thereof is formed by a flipflop circuit. As soon as the capacitor voltage exceeds one of the trigger points of the flipflop circuit, the flipflop switches over and the switching means are set to the first or the second position. A relaxation oscillator of this kind delivers a delta voltage whose peak-to-peak value is determined, interalia, by said first and second voltage values, the hysteresis of the flipflop circuit, and the loop gain present at the instant of switching. The cited article discloses, explicitly or otherwise, a number of possibilities for improving the noise behavior of the relaxation oscillators, i.e. increasing the capacitor delta voltage, increasing the loop gain at the instant of switching, reducing the effect of the hysteresis of the flipflop circuit, and the fast switching over of the current by means of a fast flipflop circuit.
The problem of noise in relaxation oscillators can usually be avoided by using other oscillator types displaying low-noise behavior. However, considering the advantages offered by a relaxation oscillator, such as a wide frequency tuning range and the possibility of integration in an IC, further improvement of the noise behavior of the relaxation oscillators is desirable.